Small organic dye molecules, especially small organometallic complexes, are well known for their high fluorescent efficiencies in electroluminescence (EL) devices. For example tris(8-quinolinolate)aluminum complex (Alq) is one of the most efficient small molecular organic electroluminescent materials. Compared to inorganic direct band gap semiconductor (such as GaAs) devices, the cost to fabricate those small molecule organic semiconductor devices for commercial use is low. However the fabrication cost is still too high when compared to polymer semiconductor devices, since polymer film can be fabricated by simple spin-coating, or printing instead of thermal evaporation as in the case of small organic molecules. More importantly, polymer processing is suitable for large-area light-emitting displays which are more difficult to process using small organic molecules. Furthermore, polymers do not suffer from the problem of aggregation or recrystallization as small organic molecules do, which has been one of the major recognized hurdles for the long term reliability of small organic molecule semiconductor devices.
To lower the manufacturing cost and yet keep the large-area display capability of a polymer system, molecularly doped polymers have been studied for light emitting devices (LEDs) by Japanese scientists. Kido and coworkers (J. Kido, et. al., Jpn. J. Appl. Phys., 31, L961 (1992)) have used triphenyldiamine derivative (TPD)- doped poly(methyl methacrylate) and polycarbonate as hole transport layers in organic electroluminescent devices. He has achieved white light-emission from three fluorescent dye-doped poly(vinylcarbazole) in a single Light-emitting diode (LED) device (J. Kido, et. al., Appl. Phys. Lett., 64(7), 815 (1994)). In one instance, he fabricated a LED with Alq and TPD doped PMMA and obtained green emission of luminance 920 cd/m.sup.2 at 17 V (J. Kido, et. al. Appl. Phys. Lett., 61(7), 761 (1992)). Though molecularly doped polymer devices can be fabricated inexpensively and applicable to large-area LED display, they have also been plagued with the stability problems caused by the phase-separation of small molecular dopants from the polymer matrix due to the incompatibilities of small molecular dopants with the polymers.
Fluorescent organometallic complexes stand out from the rest of small organic fluorescent dyes for their semi-conductivity, thermal stability and high EL efficiency in LED applications.
It is a purpose of this invention to provide a class of new materials for use in light emitting devices that combines the high EL efficiency of small organometallic complexes with the easy processability of polymers.
It is another purpose of this invention to provide a class of new materials for use in light emitting devices with improved reliability.
It is still another purpose of this invention to provide a class of new materials for use in light emitting devices for large-area display applications.
It is yet another purpose of this invention to provide a class of new materials for use in light emitting devices which is relatively easy and inexpensive to manufacture.